Autoimmune diseases, such as ulcerative colitis (UC), are not contagious. These conditions stem from the dysregulation of a person’s immune system and cannot spread to others.

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This article examines whether UC is contagious and the factors that cause the condition. We also look at possible infectious causes of UC, other types of UC, and how a doctor will diagnose them.

The Centers for Disease Control and Prevention (CDC) estimates that roughly 3 million people in the United States have UC or Crohn’s disease, both forms of inflammatory bowel disease (IBD).

Experts consider UC an autoimmune disease. Autoimmune diseases occur when the body’s immune system becomes overactive and attacks healthy tissue. With UC, the immune system attacks the lining of the colon and rectum, causing inflammation.

Autoimmune diseases, such as UC, are not conditions people can catch from others.

Learn more about UC as an autoimmune disease.

While experts do not fully understand the exact cause, UC may develop due to several factors.

Immune system response

A person with UC may have a dysregulated immune system which causes their immune system to respond differently and mistakes the “healthy bacteria” in the gut as a threat and attacks it, causing inflammation.

Another theory is that an infection in the digestive system may trigger an inflammatory immune response. A functioning immune system will lessen the response once the infection clears. However, a dysregulated immune system may continue its response, leading to ongoing inflammation in the colon.

Both theories corroborate that UC is not contagious. Dysfunction within the immune system triggers an internal response that cannot spread to others.


There also appears to be a genetic component to the onset of UC. A person with a first-degree relative with UC is four times more likely to develop the disease.

As with other autoimmune conditions that run in the family, they occur due to the genes a person inherits, not directly through contact or exposure.

Read more about UC and genetics.

Environmental and lifestyle factors

A person’s environment and lifestyle may also influence the onset of UC. The following factors may contribute to the development of UC:

  • diet
  • stress
  • air pollution

These factors can impact people differently, as not all people will go on to develop UC, nor will the person trigger the same response in anyone else.


In a “healthy” gut, there is a balance of microorganisms, known as gut microbiota. When balanced, these microorganisms allow the gut to function properly and contribute to regulating the immune system’s function.

Experts believe that an interruption or change in microbiota balance can play a role in the onset of UC.

Similar to the immune system malfunctioning, this is an internal process that may occur in someone who cannot spread to others.

Learn more about IBD from our dedicated hub.

UC is not contagious, but bacterial, viral, and parasitic infections affecting the colon could contribute to the condition’s onset.

These infectious causes can also spread to other people, but they will not lead to the development of conditions such as UC in all cases.

A person may have exposure to these infections by:

  • drinking water that contains a contaminated substance
  • eating food that contains a contaminated substance
  • touching the stool of an affected person and then touching one’s mouth with that hand

When such an infection is present in the colon, it may trigger an immune response that causes inflammation in the affected area in some people.

Infections affecting UC

Research shows that infections caused by strains of bacteria of Salmonella and Clostridium (C.) difficile could link to the onset of UC.

Further research found that there may also be a link between types of bacteria, including strains of Escherichia coli (E. coli) and Helicobacter pylori (H. pylori), as well as viruses such as norovirus with UC.

Infectious colitis can also present as inflammatory colitis. Most infectious colitis cases resolve after approximately 1 week. However, severe symptoms may last for several weeks.

Doctors treat most infectious colitis cases with antibiotic therapy but can also prescribe other lines of treatment where relevant.

There are several different types of UC:

  • Ulcerative proctitis: With ulcerative proctitis, inflammation is within the rectum. Symptoms of this type may include:
  • Left-sided colitis: The continuous inflammation will begin in the rectum and extend to the splenic flexure. Symptoms may include:
  • Proctosigmoiditis: This affects the rectum and the sigmoid colon at the end of the colon. Symptoms may include:
    • abdominal pain and cramps
    • bloody diarrhea
    • constant urge to pass stool
  • Pancolitis, or extensive colitis: This affects the entire large intestine. Symptoms include:
    • abdominal pain and cramps
    • bloody diarrhea
    • fatigue
    • weight loss

To diagnose UC, a doctor will ask the person about their:

  • general health
  • family history
  • diet

Following this, they conduct a series of tests, including:

  • a physical exam — a doctor may check for tenderness or lumps in a person’s stomach and perform a digital rectal examination
  • blood tests to detect inflammation
  • stool sample tests can check for signs of infection in case that is what is causing colitis
  • X-rays
  • a sigmoidoscopy — a doctor will use an endoscope to examine the inflammation in the person’s lower colon and rectum
  • a colonoscopy — a doctor will use an endoscope to examine the entire colon
  • a CT scan of the pelvis or abdomen

These tests, and biopsies taken from the sigmoidoscopy or colonoscopy, will help a doctor determine whether a person has UC, what type of UC it is, and if an infection has triggered the response.

Find out more about UC diagnosis.

UC is an autoimmune disorder affecting the colon and rectum lining. Research suggests several potential causes, including genetic predisposition to the disease and environmental factors.

While people cannot spread an autoimmune condition to others, gastrointestinal infections that may influence UC are contagious to other people. People will require prompt treatment to avoid complications that may overlap with UC.

Doctors can diagnose the type of UC, but they will be unable to determine the exact cause unless an infection triggers the symptoms.